Heat the oil in a large frying pan. Add the mustard and cumin seeds and, when they start to pop, add the curry leaves, ginger, chillies and onions.
Cook over a moderately low heat for 10 minutes, stirring occasionally, until the onions are golden.
Add the pumpkin, turmeric and a little salt and mix well for one minute then pour in the coconut milk and 200 ml water. Bring to the boil, stirring constantly.
Add the prawns and cook, stirring for 5 minutes or until the prawns and pumpkin are cooked. Add the vinegar and mix well.